ENSC 14A CHAPTER 8.1

• Power cycles


  Devices or systems used to produce a net power output
• Types of power cycles

  • Gas power cycle
  • Vapor power cycle
• Gas power cycle


  The working fluid remains in the gaseous phase throughout the entire cycle
• Vapor power cycle


  The working fluid exists in the vapor phase during one part of the cycle and in the liquid phase during another part
• Otto cycle


  Model for spark ignition (SI) engines
• Diesel cycle


  Model for compression ignition (CI) engines
• Rankine cycle


  Model for vapor power cycle
• Carnot cycle, the most efficient cycle operating between a heat source and sink, is NOT a suitable model for gas power and vapor power cycles because it cannot be approximated in practice
• Refrigeration cycles


  Devices or systems used to move heat around (either hot or cold)
• Ideal vapor compression refrigeration cycle


  One of the refrigeration cycles
• Actual vapor compression refrigeration cycle


  One of the refrigeration cycles
• Power cycles


  • Operate on thermodynamic cycles
  • Produce net power output
• Refrigeration cycles


  • Operate on thermodynamic cycles
  • Move heat around (hot or cold)
• Types of power cycles based on working fluid phase

  • Gas power cycles (Otto, Diesel)
  • Vapor power cycles (Rankine)
• Types of power cycles based on working fluid circulation

  • Closed cycle (Rankine, Vapor Compression)
  • Open cycle (Otto, Diesel)
• Internal combustion engine (ICE)

  Engines where heat is supplied by burning fuel within the system boundaries
• External combustion engine (ECE)

  Engines where energy is supplied to the working fluid from an external source
• ICE engines operate on a closed gas cycle, ECE engines operate on an open vapor cycle
• Idealization and simplifications


  • No friction between surfaces in contact
  • Quasi-equilibrium processes
  • No heat transfer; heat loss or gain in pipes connecting system components is negligible
  • Changes in kinetic and potential energies of the working fluid are negligible
• Air standard assumptions


  • Air behaves as an ideal gas throughout the entire cycle
  • All processes are internally reversible
  • Substitute: heat addition for combustion
  • Substitute: heat rejection for exhaust
• Air standard cycle


  A cycle for which the air standard assumptions are applicable
• Cold air standard assumption

  Specific heat of air is constant (cp, cv evaluated at room temp: 25°C or 77°F)
• Reciprocating engine


  An engine in which one or more pistons move up and down in cylinders
• Parts of a reciprocating engine

  • Top Dead Center (TDC)
  • Bottom Dead Center (BDC)
  • Stroke
  • Bore
  • Intake valve
  • Exhaust valve
• Clearance volume


  Minimum volume formed in the cylinder when the piston is at TDC
• Displacement volume


  Volume displaced by the piston as it moves between TDC and BDC
• Compression ratio


  Ratio of the maximum volume formed in the cylinder to the minimum volume
• Mean Effective Pressure (MEP)

  A fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle
• Types of reciprocating engines

  • Spark Ignition (SI) Engines (Otto cycle)
  • Compression Ignition (CI) Engines (Diesel cycle)
• Otto cycle


  An ideal cycle for spark ignition engines
• In most SI engines, the piston executes 4 complete strokes (two mechanical cycles) within the cylinder and the crankshaft completes two revolutions for each thermodynamic cycle
• Compression Ignition (CI) Engines 


  The air-fuel mixture is ignited as a result of compressing the mixture above its self ignition temperature
• OTTO CYCLE


  • Named after Nikolaus A. Otto, who built a successful 4 stroke engine in 1876 in Germany using the cycle proposed by Frenchman Beau de Rochas in 1862
  • In most SI engines, the piston executes 4 complete strokes (two mechanical cycles) within the cylinder and the crankshaft completes two revolutions for each thermodynamic cycle
• Four Stroke Spark Ignition Engine

  1. Intake

  2. Compression

  3. Power

  4. Exhaust
• Four Internally Reversible Processesof Otto Cycle


  1 - 2 Isentropic Compression

  2 – 3 Constant Volume Heat Addition

  3 - 4 Isentropic Expansion

  4 - 1 Constant Volume Heat Rejection
• The increase in thermal efficiency with the compression ratio is not that as pronounced at high compression ratios
• When high compression ratios are used, the temperature of the air-fuel mixture rises above the auto-ignition temperature of the fuel during the combustion process, causing an early and rapid burn of the fuel
• Auto-ignition in spark ignition engines cannot be tolerated because it hurts performance and can cause engine damage
• Auto-ignition


  Premature ignition of fuel
• Engine knock


  An audible noise produced during auto-ignition